This invention relates to geothermal energy in general and more particularly to an improved method of extracting geothermal energy in a hot, dry rock system.
With the shortage of petroleum products and high prices, there is great deal of interest in alternate sources of energy. One such source is geothermal energy. This energy is energy taken from the natural heat of the earth. Various systems have been developed for such purposes. Typical are those disclosed in U.S. Pat. Nos. 3,817,038, 3,786,868 and 3,911,638. In a hot dry rock system such as that disclosed in U.S. Pat. No. 3,817,038, an injection well and a production well are drilled and a fluid is injected into a geothermal area through the injection well, the fluid forced through the formation with simultaneous heating and the heated fluid then recovered from the production well. The recovered heated fluid is then used on the surface to generate energy. For example, the heated fluid may be expanded to steam and used to drive a steam turbine, the condensate from the steam turbine along with any makeup water then being reinjected in the injection well to form a closed system.
Another approach similar to the one in which two wells are drilled is one in which a single well is completed with a dual casing string which permits injection of cold water at the bottom of the fracture system and recovery of hot water at the top of the fracture. A third approach, which is known as "huff and puff", is one in which the well is operated in a pulsed mode where water is alternately pumped into the fracture, allowed to heat up and then withdrawn. Operation of several wells of this type in sequence provides power for sustained operation. The pulsed mode operation has the additional virtue of permitting use for load following applications, i.e., for driving a generator which follows the electrical demand load, pulsing can be controlled in dependence on the demand.
Injection and production of water requires energy and any approach which diminishes the reinjection or production energy required, diminishes the cost of producing geothermal energy from this resource and increases the net amount of energy recovered from the resource reducing waste and increasing the net reserves of energy indigenous to the United States.